Atmospheric aging of naturally emitted marine aerosol often leads to formation of internally mixed particles composed of sea salts and water-soluble organic compounds of anthropogenic origin. Mixing of sea salt and organic components has profound effects on the evolving chemical composition and hygroscopic properties of the resulted particles, which are poorly understood. Here, we have studied chemical composition and hygroscopic properties of laboratory generated NaCl particles mixed with malonic acid (MA) and glutaric acid (GA) at different molar ratios using micro-FTIR spectroscopy, atomic force microscopy, and X-ray elemental microanalysis. Hygroscopic properties of internally mixed NaCl and organic acid particles were distinctly different from pure components and varied significantly with the type and amount of organic compound present. Experimental results were in a good agreement with the AIM modeling calculations of gas/liquid/solid partitioning in studied systems. X-ray elemental microanalysis of particles showed that Cl/Na ratio decreased with increasing organic acid component in the particles with MA yielding lower ratios relative to GA. We attribute the depletion of chloride to the formation of sodium malonate and sodium glutarate salts resulted by HCl evaporation from dehydrating particles.

Scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) and optical microscopy coupled with Fourier transform infrared spectroscopy (micro-FTIR) have been applied to observe hygroscopic growth and chemical changes in malonic acid particles deposited on substrates. The extent of the hygroscopic growth of particles has been quantified in terms of the corresponding water-to-solute ratios (WSR) based on STXM/NEXAFS and micro-FTIR data sets. WSR values derived separately from two applied methods displayed a remarkable agreement with previous data reported in the literature. Comparison of NEXAFS and FTIR spectra acquired at different relative humidity (RH) shows efficient keto-enol tautomerization of malonic acid, with the enol form dominating at higher RH. The keto-enol equilibrium constants were calculated using relevant peak intensities in the carbon and oxygen K-edge NEXAFS spectra as a function of RH. We report strong dependence of the equilibrium constant on RH, with measured values of 0.18 ± 0.03, 1.11 ± 0.14, and 2.33 ± 0.37 corresponding to 2, 50, and 90% RH, respectively. Enols are important intermediates in aldol condensation reactions pertaining to formation and atmospheric aging of secondary organic aerosol (SOA). The present knowledge assumes that constituents of atmospheric deliquesced particles undergo aqueous chemistry with kinetic and equilibrium constants analogous to reactions in bulk solutions, which would estimate absolute dominance of the keto form of carboxylic acids. For instance, the keto-enol equilibrium constant of malonic acid in diluted aqueous solution is <10(-4). Our results suggest that in deliquesced micrometer-size particles, carboxylic acids may exist in predominantly enol forms that need to be explicitly considered in atmospheric aerosol chemistry.

A novel application of single particle scanning transmission X-ray microscopy (STXM) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy is presented for quantitative analysis of hygroscopic properties and phase transitions of individual submicrometer particles. The approach utilizes the exposure of substrate-deposited individual particles to water vapor at different relative humidity followed by STXM/NEXAFS spectromicroscopy analysis. The hygroscopic properties of atmospherically relevant NaCl, NaBr, NaI, and NaNO(3) submicrometer particles were measured to evaluate the utility of the approach. An analytical approach for quantification of a water-to-solute ratio within an individual submicrometer particle during hydration and dehydration cycles is presented. The results for the deliquescence and efflorescence phase transitions and quantitative measurements of water-to-solute ratios are found in excellent agreement with available literature data. Oxygen K-edge NEXAFS spectra of submicrometer sodium halide droplets are reported along with a unique experimental observation of the formation of the halide-water anionic complex in NaBr and NaI microdimensional droplets. The analytical approach provides a unique opportunity for spectromicroscopy studies of water uptake on environmental particles collected in both laboratory and field studies.

Metallogels form from Cu(II) ions and tetratopic ligand rctt-1,2-bis(3-pyridyl)-3,4-bis(4-pyridyl)cyclobutane. The tetrapyridyl cyclobutane has been synthesized in the organic solid state. The gel forms with a variety of counteranions and gels water. The hydrogel is thixotropic and is composed of nanoscale metal-organic particles (NMOPs), a high surface area of which likely accounts for the gelation of the polar aqueous medium. A shear stress profile of the thixotropic hydrogel gave a yield value of 8.33 Pa. A novel combination of atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM) is used to assess the densities of individual NMOPs. A density of 1.37 g/cm(3) has been determined. A single-crystal X-ray diffraction study demonstrates the ability of the unsymmetrical cyclobutane 3,4'-tpcb to self-assemble with Cu(II) ions in [Cu(2)(hfac)(4)(3,4'-tpcb)](∞) (where hfac is hexafluoroacetylacetonate) to form a solvated 1D coordination polymer.

A correlation between Young's modulus, as determined by using nanoindentation atomic force microscopy (AFM), and atomic polarizability is observed for members of a series of cocrystals based on systematic changes to one cocrystal component. Time domain spectroscopy over terahertz frequencies (THz-TDS) is used for the first time to directly measure the polarizability of macro- and nanosized organic solids. Cocrystals of both macro- and nanodimensions with highly polarizable atoms result in softer solids and correspondingly higher polarizabilities.